This invention relates to the metallothermic preparation of metals and more particularly is concerned with a novel process for the production of magnesium metal by the metallothermic reduction of magnesium oxide at high temperatures in the presence of an aluminum silicon alloy reducing agent and a molten oxidic slag in an electric furnace and the condensation and recovery of vaporized magnesium in a condenser.
Thermal reduction processes for the conversion of magnesium oxide or of substances containing magnesium oxide to metallic magnesium have evolved along two general lines: those which use carbon as a reducing agent (i.e., carbothermic processes) and those which use free metals as a reducing agent (i.e., metallothermic processes). In both types of processes the necessary heat of reaction is usually supplied by an electric arc furnace in which an electric current may be passed through the feedstock mixture and usually is passed through the resulting liquid or solid slag by-product. It is known that aluminum is a very effective free-metal reducing agent for magnesium oxide. It is also known that aluminum can be obtained relatively cheaply in the form of an aluminum silicon alloy, for example, by carbothermic smelting of aluminum-silicate ores. Examples of known processes for the production of magnesium wherein an aluminum-silicon alloy is used as the metallic reducing agent are described below.
U.S. Pat. No. 3,579,326 teaches a process for producing magnesium by reducing magnesium oxide from an oxidant containing a major proportion of magnesia (rather than dolomitic lime) with a metallic aluminum-silicon alloy reductant having a ratio of silicon to aluminum of at least 0.8 to 1.0 (i.e., of at least about 40 percent Si) at a temperature of at least 1400.degree. C. and at a pressure of about 1 atmosphere in the presence of a molten slag containing 15 to 35 percent alumina, less than 30 percent calcium oxide, 5 to 25 percent magnesium oxide, and 25 to 50 percent silica. The molecular ratio of magnesium oxide to calcium oxide in the oxidant of the U.S. Pat. No. 3,579,326 process is at least 2:1. The ratio of aluminum and magnesium oxides to silicon dioxide in the slag is less than 1.6, the aluminum and magnesium oxides comprise less than 50 percent of the slag, and the ratio of calcium oxides to the silicon dioxide of the slag is less than 1.6. Generally the patent teaches that:
The composition of slag is determined by the ratio of aluminum to silicon fed as the reducing agent, the degree of utilization of silicon as reductant, which for reasons of economy should be as high as feasible; and the relative proportion of magnesium oxide fed as magnesia and as dolomitic lime. (Column 4, lines 38-43) (emphasis added) PA1 Finally, there is the question of aluminum itself as an impurity in the magnesium product, since aluminum has, a vapor pressure of about 10 mm. Hg at 1500.degree. C. This means that magnesium produced by the present process will inevitably contain aluminum--how much depends upon the operating temperature. At 1400.degree. C., for example, it would contain about 0.5 percent, at 1500.degree. C. about 1.2 percent and at 1600.degree. C., about 2 percent of aluminum. However, this is not a serious problem and may in fact be beneficial, because: (a) the principal use for magnesium today is to produce aluminum alloys for fabrication; and (b) a major portion of the magnesium used for fabricated magnesium products contains a substantial proportion of aluminum--generally from 3 to 9 percent. Thus the presence of a small amount of aluminum in the magnesium produced by the present process is not detrimental, especially if the magnesium operation is associated with the production of aluminum, which is likely to be the case because of the advantage of recovering Al.sub. 2 O.sub.3 from the slag produced, and the possibility of using captive scrap as the reducing agent.
The examples summarized in Table I of said patent show production of a ferrosilicon alloy by-product containing from 56 to 75 percent Si when the metallic aluminum silicon alloy reductant also contains iron, but show no production by-product alloy when iron is not present in the reductant.
U.S. Pat. No. 3,782,922 teaches a process for producing magnesium by reducing magnesium oxide from an oxidant containing a major proportion of magnesium oxide (the weight ratio of MgO:CaO in the oxidant is between about 1.1 and 2.3) with a substantially pure aluminum reductant (i.e., the reductant contains at least 85 percent aluminum) at a temperature between about 1300.degree. C. and 1700.degree. C. and at a pressure of about 1 atmosphere wherein the slag produced as a by-product of the reaction has a composition of about 35-65 percent alumina, 35-55 percent calcium oxide, 0-10 percent silica, and less than 5 percent magnesia (when the slag is removed from the system). The particular calcium aluminate slag produced by this process is said to be highly advantageous in that alumina can readily be recovered by leaching the slag with Na.sub.2 CO.sub.3 solution. Among the asserted advantages of the U.S. Pat. No. 3,782,922 process is the virtually complete consumption of the aluminum reducing agent in the primary reaction, which avoids the necessity of recycling or disposing of considerable quantities of spent metal (Column 4, lines 69-74). However it is noted at the bottom of Table III appearing in Column 11 of the specification that "The magnesium produced will contain up to 20 percent Al." Compare the following statements appearing at Column 3, lines 24-45 of the same specification:
Whatever the specific aluminum content of the magnesium produced by the U.S. Pat. No. 3,782,922 process may be, it is clear that the product will contain a significant amount of aluminum. Furthermore, since nearly all of the magnesium oxide present in the oxidant charge is converted to metallic magnesium vapor in the reduction furnace (the magnesia content of the slag is less than 5 percent, preferably less than 2 percent), it is apparent that, according to the U.S. Pat. No. 3,782,922 teaching, the quantity of aluminum reductant fed to the furnace may be somewhat greater than the stoichiometric amount. This is expected because of relatively high vapor pressure of aluminum metal at the process temperature and the consequent carry-over of aluminum vapor with the volatile magnesium product. Nevertheless, the U.S. Pat. No. 3,782,922 process does not produce a by-product spent alloy reductant (as noted above). FIG. 1 of the patent does show removal of a "metallic residue" from the furnace, but that "residue" refers to "impurities" such as copper or chromium which are present in certain alloys and scraps which may be employed as the "substantially pure aluminum" reductant in the process (see Column 8, lines 43-56).
U.S. Pat. No. 4,033,758 teaches a process for producing magnesium by reducing magnesium in a calcium magnesium aluminum silicate slag or magnesium oxide in the presence of such a slag with a metallic aluminum silicon alloy reductant comprising from 15 to 75 percent by weight aluminum and from 20 to 80 percent by weight silicon at a temperature of about 1400.degree. to 1650.degree. C. and a pressure of about 25 to 500 mm of Hg (about 0.03 to 0.66 atmosphere) in the presence of a molten slag containing 11 to 38 percent alumina, 42 to 65 percent calcium oxide, 1 to 11 percent magnesium oxide, and 5 to 19 percent silica. The amount of magnesium oxide fed to the reaction zone is at least 101 percent of the amount theoretically required to react with the aluminum silicon alloy reductant.
U.S. Pat. No. 4,033,759 teaches a process for producing magnesium by reducing magnesium in a calcium magnesium aluminum silicate slag or magnesium oxide in the presence of such a slag with metallic aluminum reductant containing at least 80 weight percent aluminum at a temperature of about 1350.degree. to 1700.degree. C. and a pressure of about 0.5 to 2.0 atmospheres in the presence of a molten slag containing 28 to 64 percent alumina, 30 to 65 percent calcium oxide, 6 to 13 percent magnesium oxide, and less than 5 percent silica. The amount of magnesium oxide fed to the reaction zone is at least 110 percent of the amount theoretically required to react with the aluminum metal reductant. The patent suggests that it is essential to keep the silica concentration in the furnace at a low value (less than 5 weight percent) when using aluminum metal as a reducing agent because of the undesirable side reaction of aluminum with silica to produce by-product silicon metal (Column 3, lines 42 to 63). The combination of reactants taught by the U.S. Pat. No. 4,033,759, particularly the high concentration of aluminum metal in the reductant and the stoichiometric excess of magnesium oxide, is said to provide a superior process in that the reaction between aluminum metal and magnesium readily occurs and a high utilization of expensive aluminum metal is obtained.
U.S. Pat. No. 3,441,402 claims a metallothermic method for the production of magnesium in a submerged arc furnace wherein an oxidant mixture of calcined dolomite and a second magnesium ore selected from the group consisting of calcined magnesite and dried serpentine is reduced with a reducing agent at temperatures below about 1500.degree.C. and pressures of about 1 atmosphere. The patent teaches the use of aluminum-silicon alloys containing from about 30 to 100 weight percent aluminum as a reducing agent in the process (Column 3, lines 32-35). The Examples suggest the addition of aluminum-silicon alloy in approximately stoichiometric amounts: the operations of Example 1 show a slight (6-13%) excess of silicon-aluminum alloy (which alloy contains 66.34 weight percent aluminum) and Example 2 also suggests an excess of silicon-aluminum alloy. The Examples further suggest that the reduction of the mixture of magnesium ores occurs in the presence of a molten slag containing about 20 to 30 percent alumina, 30 to 50 percent calcium oxide, less than 10 percent magnesium oxide, and 15 to 40 percent silica.
Of the five foregoing patents describing metallothermic processes for the conversion of magnesium oxide or of substances containing magnesium oxide to metallic magnesium using an aluminum-silicon alloy as reductant, three of the patents teach or suggest the addition of less than the stoichiometric amount of magnesium oxide theoretically required to react with the metallic reductant: U.S. Pat. Nos. 3,579,326; 3,782,922; and 3,441,402. However, none of these three patents teach or suggest the addition of less than the stoichiometric amount of magnesium oxide theoretically required to react with the aluminum component of an aluminum-silicon alloy reductant--with the possible exception of U.S. Pat. No. 3,782,922 (which, it should be noted, employs a "substantially pure aluminum reductant"). The excess addition of reductant in the U.S. Pat. No. 3,782,922 process is necessary to compensate for reduction losses caused by the relatively high vapor pressure of the reductant at process temperatures. Accordingly, none of the patents specifically teach or suggest the addition of less than the stoichiometric amount of magnesium oxide theoretically required to react with the aluminum component of an aluminum-silicon alloy reductant present in a magnesium reduction zone. Furthermore, none of the patents specifically teach or suggest the addition of less than the stoichiometric amount of magnesium oxide theoretically required to react with aluminum combined with the retention of unreacted aluminum in the by-product alloy; none of the patents teach or suggest the addition of sufficiently less than stoichiometric amounts of magnesium oxide so as to have aluminum remaining in a by-product alloy.
It may be noted, though, that U.S. Pat. No. 2,847,295 does teach the addition of a surplus of metallic reductant--an amount exceeding the theoretically necessary amount to completely reduce magnesium oxide contained in an oxidant feed--and more particularly teaches that when the reductant component acting as the reduction material is added with accompanying metals, the surplus of the reductant component should be such that compounds or alloys of the component with the accompanying metals will remain after the reaction is completed. The object of the surplus addition is to minimize the deleterious effect of the compounds or alloys on the reaction capability of the reductant component acting as the reduction material and to maintain an "almost metallic conductivity"in shaped bodies comprising the reaction medium until the end of the reaction. Unlike the hereinbefore described process, the U.S. Pat. No. 2,847,295 process does not reduce magnesium oxide in the presence of a molten oxidic slag. Rather, the reactant and residual materials are maintained in the solid state without the appearance of a liquid phase (see Column 2, lines 11-15 and Column 4, lines 7-23).
Another process of interest (although it does not teach the use of an aluminum silicon alloy reductant) is disclosed in British Pat. No. 922,300 which is a modified aluminothermic process called the "MC process". The process described therein is a two-stage cyclic process for the production of magnesium. In the first stage, a material containing magnesium oxide and lime is reduced by means of aluminum to form magnesium metal vapor and a calcium aluminate slag. In the second stage, the calcium aluminate slag is reacted with carbon in the presence of an auxilliary metal selected from the group consisting of iron and copper to form calcium carbide, a residual slag, and an alloy of aluminum metal with the auxilliary metal, which alloy is returned to the first stage for the production of further quantities of magnesium metal. The auxilliary metal thus serves as a carrier metal for the aluminum metal reductant. Like the teachings previously discussed, this patent neither shows nor suggests the addition of less than the stoichiometric amount of magnesium oxide theoretically required to react with the aluminum component of an aluminum alloy reductant; the Example shows the addition of an excess of about 14 percent magnesium oxide.